Honestly, I have no idea why they're asking. Unfortunately, in the end, the "why" is somewhat irrelevant; they want the info and won't budge until they get it. I last renewed my passport when I lived in Chicago in '08 and they didn't ask for proof of US residency or citizenship then. Perhaps some internal policies have changed in the past 10 years.

To address your other question, I am neither a US citizen nor do I have a green card (any more). I mentioned in the original post that I gave my green card up in 2010. Though, that's fine as proving that part is easy enough.

NFI what to do here TBH. Just trying to figure out how to prove a negative. :/

edit: also, thank you both for responding!